Scintillation phosphors, infrared and ultraviolet radiation transmitting shaped articles are used in a host of applications. For example, a scintillation phosphor coupled to a photomultiplier tube is used for the detection of ionizing radiation in devices ranging from simple scintillation counters to sophisticated camera plates for medical use in connection with the analysis of gamma radiation emanating from patients who are injected with specific isotopes. Windows transparent to radiation, in the range from microwave through infrared and ultraviolet radiation, are used in instruments of various kinds, as well as for domes and windows in missiles and related devices.
In the applications mentioned hereinbefore, as well as in many others, shaped articles transparent to the aforementioned wave length regions have been limited in size by the peculiar physical properties of ionic crystals. The demanding requirements of windows, domes, and lenses large enough for modern commercial and military requirements are antithetical to the well-known size limitation of shaped articles made from ionic crystals. Previously, acceptable windows have been pressed from single crystal fragments whose average particle size has ranged from less than 10 microns to several millimeters. Larger shaped articles have been prepared from hot-pressed polycrystalline calcium fluoride and other alkali metal and alkaline earth metal halides. Hot-pressed materials, however, made for example as described in U.S. Pat. No. 3,359,066 have markedly inferior properties are compared to lenses, domes, and windows made from single crystals, or macrocrystal ingots artificially melt-grown, at this time, in sizes up to about 30 inches in diameter and about 1 foot long, wherein individual crystal grains may range in size up to a nominal diameter of about 8 inches or more. Thus, as of the present time, where a high quality, shaped article is required, whether it is for use as a scintillation phosphor, laser window, or an infrared or ultraviolet transmitting lens, when a relatively large shaped article is required, several individual single crystal pieces, or sections from a macrocrystal ingot are individually sawed off the melt-grown ingot and then adhesively bonded together to form the larger shaped composite. Notwithstanding the arduous devotion which is a perquisite of successful fabrication of a relatively large shaped article having superior radiation transmission properties, the end result was an article of determined fragility and transmission properties so marginally superior to those of hot-pressed articles as to negate the use of such relatively large composites for all but those applications where cost is not a factor. Moreover, a composite formed in this manner suffers from the drawbacks of degradation of light output due to the optical interfaces. No matter how carefully the faces of sections are polished before they are bonded into a composite, there is no way of eliminating the undesirable effects of the interface.
In concurrently filed application Ser. No. 180,087 is described the formation of scintillation phosphors formed as extrudates of unrestricted length and arbitrary cross sections by extrusion of a single crystal or macrocrystal ingot at a temperature below its melting point and under sufficient pressure to form a coherent, homogeneous, fully dense polycrystalline material.
Extrusion of a sodium chloride, single crystal billet fitted tightly into an extrusion chamber and forced through an extrusion die maintained at various temperatures above 300.degree. C. to yield a rod of polycrystalline sodium chloride which was completely clear and free from porosity, is known. (see "Mechanical Properties of Polycrystalline Sodium Chloride" by R. J. Stokes, Proceedings of the British Ceramic Society, Vol. 6, page 192, June 1966). This work was done in connection with the mechanical properties of polycrystalline sodium chloride in relation to those of polycrystalline magnesium oxide which had a similar lattice structure but a melting point of about 2650.degree. C., which is so high as to make the direct study of this "more technologically significant material" all but impossible. Reason for the choice of sodium chloride, other than from the technological aspect of understanding and improving the strength of ceramics and the fundamental aspect of understanding the role of grain boundaries and the deformation of solids as a whole, is that it is an ionic solid which is transparent and affords the opportunity for examining grain-boundary interfaces within the solid rather than their intersection with an external surface, as is the case with opaque materials; and, that being a non-metallic solid, it possesses a wide range of crystal structures and shows a wide variety of slip parameters. Ionic solids thus provide a greater choice of materials on which possible correlations between slip mode and polycrystalline deformability can be examined. Stokes et al. found that sodium chloride conforms to von Mise's criterion, that is to say, for polycrystalline deformation, the slip parameters of a solid should lead to five independent slip systems. Stokes et al. note, however, that the analysis is based upon macroscopic plasticity theory and neglects the discrete nature of the slip process, and "even when slip is homogeneous, and occurring on a number of planes as at higher temperatures, interpenetration of slip is limited by interaction between the various slip dislocations. Localized work-hardening then restricts further deformation, and again, although the crystal has the requisite number of slip systems, the ideal situation assumed in the analysis does not necessarily prevail." (p. 191) They concluded that "insofar as slip mode is concerned, the plasticity of polycrystalline rock salt depends on three important factors:
1. The ability for microscopic cross slip out of the (110) plane. PA1 2. The number of independent slip systems. PA1 3. The degree to which different slip systems interpenetrate. PA1 4. Grain boundary sliding. PA1 5. Polygonization and recrystallization of severely deformed regions."
These three factors taken together constitute what has been referred to as slip flexibility. At high temperatures two additional factors contribute to polycrystalline plasticity:
In view of these findings, it is unexpected and surprising that a polycrystalline, fully dense solid having a relatively small grain size may be re-extruded at a temperature below its melting point, yet maintain its scintillation properties and its ultraviolet and infrared radiation transmitting capacity, essentially the same as, and in some cases better than, the archetype single crystal or macrocrystal ingot.
At an earlier date, in an attempt to extend the study of single crystals to polycrystalline specimens, it was found that conventional fabrication techniques introduced not only grain boundaries but also impurities and porosity in variable amounts. It was also found that specimens of high purity and high degree of mechanical perfection must be used to obtain useful results. (Budworth, D. W., and Pask, J. A., trans. Brit. Ceram. Soc., Vol. 62, page 764, 1963) In a study on the plasticity of lithium fluoride, Budworth and Pask observed that "wavy slip traces on the faces of the specimens tested at 400.degree. C. and 500.degree. C. indicate that slip is no longer restricted to one set of parallel planes. It would thus seem that not until both families of slip systems (making a total of 12 possible slip modes in the crystal) can act at about the same resolved sheer stress, can the grains accommodate conveniently the strains occurring in their neighbors in accordance with the five slip system condition to produce significantly ductile behavior before fracture. This is in excellent agreement with the theoretical prediction. The increasing tendency to fail at grain boundaries as the temperature is raised may be due to either or both of two causes: (1) decreases grain boundary strength and (2) decreased ease of cleavage." (page 769) With all the movement required in the general deformation of a crystal, it is surprising that despite re-extrusion of a polycrystalline material, the scintillation properties, microwave, ultraviolet and infrared transmission properties of the polycrystalline material are maintained essentially intact despite the re-extrusion.
In still another study entitled "Effective Temperature Under Deformation of KCl-KBr Alloys" by Stoloff, Lezius and Johnston (Journal of Applied Physics, Vol. 34, No. 11, page 3315), it was shown that polycrystalline potassium chloride tested in tension undergoes a brittle-toductile fracture transition near 250.degree. C. Ductile fraction in KCl is associated with the disappearance of planar glide. However, at sufficiently high temperature, grain boundary sliding leads to premature grain boundary fracture. The addition of 0.6 percent KBr to KCl appears to influence only the strain-hardening rate, but an alloy containing 1.3 percent KBr is considerably stronger, and remains brittle to 350.degree. C. Since, in the instant invention, re-extrusion is carried at a high temperature, preferably close to but below the melting point of the polycrystalline mass, it is unexpected and surprising that grain boundary sliding does not lead to premature grain boundary fracture. Doping of the polycrystalline mass to be re-extruded is usually at a concentration so low as not to affect the melting point of the pure polycrystalline material. No apparent influence on the strain-hardening rate is observed in the re-extrudate.
As has been stated hereinbefore, it is known that a large cleaved single crystal block of sodium chloride may be water-machined into a billet which fits tightly into an extrusion chamber and then extruded through a die maintained at temperatures above 300.degree. C. to obtain a completely clear polycrystalline rod. As the temperature increases and microscopic cross slip becomes easier, the strain, and therefore the stress concentration, associated with the single slip band is reduced but the rate of hardening is high. "From the macroscopic plasticity point of view, the high rate of hardening may be regarded as a direct consequence of the incompatibility of adjacent grains. Because the grains cannot deform plastically without destroying coherence at the grain boundaries, the total plastic strain is limited to be of the same order of magnitude as the elastic strain. As the grain size increases, the compatibility requirements in the vicinity of the grain-boundary interface have relatively less influence over the remaining volume of material, permitting a greater amount of plastic strain." (Stokes, "Mechanical Properties of Polycrystalline Sodium Chloride" page 203) Despite the fact that the grains deform and can do so only by destroying coherency at the grain boundaries, the re-extruded shaped bodies of the instant invention are transparent to laser beams, i.e., permit transmission of a beam without destroying its coherency. Again, despite the general deformation of the crystal and destruction of plastic strain to which the polycrystalline first extrusion is subjected, the re-extrusion, or second extrusion, and subsequent further re-extrusions surprisingly maintain essentially the same scintillation characteristic, laser, ultraviolet and infrared radiation transmission capabilities as the original polycrystalline extrudate, which in turn maintains essentially the same optical properties as those of the original single crystal or melt-grown crystal ingot.
It is well known that relatively small crystal ingots, smaller than about one foot in diameter and about one foot high, may be inculcated with superior optical properties which cannot be emulated in larger crystals. The instant invention makes possible the formation of top-quality polycrystalline optical bodies much larger than the crystal ingot from which it is formed, by extruding the ingot at least twice. The direction of re-extrusion of a first extrudate is not critical any angle being operable. From a practical point of view maximum homogeneity is usually obtained by the directional axis of re-extrusion being at right angles to, or orthogonal to the axis of the first extrusion. For particular geometries such as long rods where superior homogeneity is desired a second extrusion in a direction parallel to the first may be used.